Add the potatoes to a large pot of cold water. Season well with salt and bring to a boil. Simmer until tender, 15 to 20 minutes. Meanwhile, add the pancetta and onions to a large frying pan.
Turn on the heat to medium-high, and saute until the pancetta is crisp and golden. Add the milk and season with salt and pepper. Heat until hot. Drain the potatoes well in a colander. Use a food mill to rice the potatoes into the milk mixture. Fold together the potatoes with the hot milk, olive oil and chopped parsley.